1. Field of the Invention
The present invention relates to a structure of an electronic device. In particular, the present invention relates to an active matrix electronic device having a thin film transistor (TFT) formed on an insulating body.
2. Description of the Related Art
Flat panel displays have been drawing attention in recent years as substitutes for LCDs (liquid crystal displays), and research into such displays is proceeding apace.
LCDs can roughly be divided into two types of driving methods. One is a passive matrix type using an LCD such as an STN-LCD, and the other is an active matrix type using an LCD such as a TFT-LCD. EL displays can also be similarly broken down roughly into two types; one a passive type, and the other an active type.
For the passive type, wirings which become electrodes are arranged in portions above and below EL elements. Voltages are applied to the wirings in order, and the EL elements turn on due to the flow of an electric current. On the other hand, each pixel has a TFT with the active matrix type, and a signal can be stored within each pixel.
A schematic diagram of an active matrix EL display device is shown in FIGS. 19A and 19B. FIG. 19A is a schematic diagram of an entire circuit, and the circuit has a pixel portion 1853 in its center. Gate signal line driver circuits 1852 for controlling the gate signal lines are arranged to the left and right of the pixel portion. The arrangement may also be on only one side, left or right, but it is preferable to use both positions as shown in FIG. 19A considering such reasons as operational efficiency and reliability. A source signal line driver circuit 1851 for controlling source signal lines is arranged above the pixel portion. A circuit for one pixel in the pixel portion 1853 of FIG. 19A is shown in FIG. 19B. Reference numeral 1801 denotes a TFT which functions as a switching element during write in of a signal to the pixel (hereafter referred to as switching TFT) in FIG. 19B. Reference numeral 1802 denotes a TFT (hereafter referred to as EL driver TFT) which functions as an element (electric current control element) for controlling electric current supplied to EL elements 1803. From the fact that it is good for TFT operation to have a source region connected to ground, and from limitations on the fabrication of the EL elements 1803, p-channel TFTs are used as the EL driver TFTs. A general structure in which the EL driver TFT is arranged between an anode of the EL element 1803 and an electric current supply line 1807 is often employed. Reference numeral 1804 denotes a storage capacitor for storing a signal (voltage) input from a source signal line 1806. One terminal of the storage capacitor 1804 of FIG. 19B is connected to the electric current supply line 1807, but a special-purpose wiring may also be used. A gate terminal of the switching TFT 1801 is is connected to a gate signal line 1805, and a source terminal of the switching TFT 1801 is connected to the source signal line 1806. Further, a drain terminal of the EL driver TFT 1802 is connected to the anode or a cathode of the EL element 1803, and a source terminal of the EL driver TFT 1802 is connected to the electric current supply line 1807.
The EL elements have a layer (hereafter referred to as an EL layer) containing an organic compound in which electroluminescence (luminescence generated by application of an electric field) is obtained, the anode, and the cathode. There is emission of light in the organic compound when returning to a base state from a singlet excitation state (fluorescence), and when returning to a base state from a triplet excitation state (phosphorescence), and it is possible to apply both types of light emission with the present invention.
Note that all layers formed between the anode and the cathode are defined as EL layers throughout this specification. These layers include, specifically, layers such as a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer. Specifically, EL element may have a structure in which an anode, a light emitting layer, and a cathode are laminated in order. In addition, structures such as one in which an anode, a hole injecting layer, a light emitting layer, and a cathode, and one in which an anode, a hole transporting layer, a light emitting layer, and an electron transporting layer are laminated in order, may also be used.
Further, elements formed by an anode, an EL layer, and a cathode are referred to as EL elements throughout this specification.
Circuit operation of the active matrix electronic device is explained next with reference to FIGS. 19A and 19B. First, the gate of the switching TFT 1801 opens when the gate signal line 1805 is selected, a voltage is applied to a gate electrode of the switching TFT 1801, and the switching TFT 1801 is placed in a conducting state. The signal (voltage) of the source signal line 1806 is thus stored in the storage capacitor 1804. The voltage of the storage capacitor 1804 becomes a voltage VGS between the gate and the source of the EL driver TFT 1802, and therefore the electric current, which responds to the storage capacitor 1804 voltage, flows in the EL driver TFT 1802 and in the EL element 1803. As a result, the EL element 1803 turns on. The brightness of the EL element 1803, namely the amount of electric current flowing in the EL element 1803, can be controlled by VGS. VGS is the voltage stored in the storage capacitor 1804, and is the signal (voltage) to be input to the source signal line 1806. In other words, the brightness of the EL element 1803 is controlled by controlling the signal (voltage) to be input to the source signal line 1806. Finally, the gate signal line in 1805 is unselected, the gate of the switching TFT 1801 closes, and the switching TFT 1801 is placed in a non-conducting state. The electric charge stored in the storage capacitor 1804 continues to be stored at this point. VGS is therefore stored as is, and the electric current in response to VGS continues to flow in the EL driver TFT 1802 and in the EL element 1803.
Information regarding the above explanation is reported upon in papers such as the following: “Current Status and Future of Light-emitting Polymer Display Driven by Poly-Si TFT”, SID99 Digest, p. 372; “High Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driver”, ASIA DISPLAY 98, p. 217; and “3.8 Green OLED with Low Temperature Poly-Si TFT”, Euro Display 99 Late News, p. 27.
Along with high definition, large size screens are sought after with active matrix EL displays. However, the increase in the length of wirings which accompanies making larger screens becomes a cause of problems such as insufficient write in time and dispersion in the electric current supplied. In particular, the dispersion in the amount of electric current supplied to the EL element due to the resistance of the electric current supply lines is directly connected with display irregularities such as uneven brightness within the screen and crosstalk, and therefore is a burden to making screens larger.
A method of reducing the resistance in the electric current supply lines for each pixel by increasing the number of electric current supply lines can be given as a way of solving the above problem. However, simply increasing the number of wirings in the pixel portion or increasing the cross sectional area of the wirings invites a reduction in the aperture ratio, and cannot be said to be a desirable method.
A novel pixel structure not found conventionally is thus sought after in order to reduce the wiring resistance while maintaining a high aperture ratio.